Overview
Combined cycle power plants and cogeneration facilities utilize gas turbines (GT(s)) as prime movers to generate power. These GT engines operate on the Brayton Cycle thermodynamic principle and typically have high exhaust flows and relatively high exhaust temperatures. These exhaust gases, when directed into a heat recovery boiler (typically referred to as a heat recovery steam generator (HRSG)), produce steam that can be used to generate more power and/or provide process steam requirements. For additional power production the steam can be directed to a steam turbine (ST) that utilizes the steam to produce additional power. In this manner, the GT produces work via the Brayton Cycle, and the ST produces power via the Rankine Cycle. Thus, the name “combined cycle” is derived. In this arrangement, the GT Brayton Cycle is also referred to as the “topping cycle” and the ST Rankine Cycle is referred to as the “bottoming cycle,” as the topping cycle produces the energy needed for the bottoming cycle to operate. Thus, the functionality of these cycles is linked in the prior art.
Rankine Cycle
Steam has been used for power applications for more than a century. Early applications utilized a pump to bring the water up to the desired pressure, a boiler to heat the water until it turned to steam, and a steam engine, typically a piston type engine, to produce shaft horsepower. These power plants were used in factories, on locomotives, onboard steamships, and other power applications.
As technology progressed, the trend for the use of steam engines diminished and the use of steam turbines increased. One advantage of the steam turbine was its overall cycle efficiency when used in conjunction with a condenser. This allowed the steam to expand significantly beyond normal atmospheric pressure down to pressures that were only slightly above an absolute vacuum (0.5 to 2 pounds per square inch absolute (psia)). This allowed the steam to expand further than in an atmospheric exhaust configuration, extracting more energy from a given mass of steam, thus producing more power and increasing overall steam cycle efficiency. This overall steam cycle, from a thermodynamic perspective, is referred to as the Rankine Cycle.
FIG. 1 illustrates the thermodynamic operation of the Rankine Cycle. In FIG. 1, graph (100) illustrates the Rankine Cycle on a Pressure versus Volume plot. From point (101) to point (102), water is pressurized at constant volume. From point (102) to point (103), the water is boiled into steam at constant pressure. Point (103) to point (104) defines the process where the steam expands isentropically and produces work. Then, from point (104) to point (101) the low-pressure steam is condensed back to water and the cycle is complete.
Also in FIG. 1, graph (110) illustrates the Rankine Cycle on a Temperature versus Entropy plot. From point (111) to point (112), water is pressurized. From point (112), the water is boiled into steam at constant temperature until it is all steam, then it is superheated to point (113). Point (113) to point (114) defines the process where the steam expands isentropically and produces work. From point (114) to point (111) the low-pressure steam is condensed back to water at constant temperature to complete the cycle. See Eugene A. Avallone and Theodore Baumeister III, MARKS' STANDARD HANDBOOK FOR MECHANICAL ENGINEERS (NINTH EDITION) (ISBN 0-07-004127-X, 1987) in Section 4-20 for more discussion on the Rankine Cycle.
Power Plant Cycle
For a number of decades, the Rankine Cycle has been used to produce most of the electricity in the United States, as well as in a number of other countries. FIG. 2 illustrates a schematic of the basic Rankine Cycle, with the four primary components being the Boiler Feed Pump (BFP) (201), Boiler evaporator/superheater (BOIL) (203, 205), Steam Turbine (ST) (207), and the Condenser (COND) (209). Note that either one or multiples of any component are possible in the arrangement, but for simplicity, only one of each is shown in FIG. 2. The sub-critical Rankine Cycle (steam pressures less than 3206.2 psia) starts as water at the inlet (211) of the BFP (201). The water is then pumped to a desired discharge pressure by the BFP (201). This pressurized water (202) is then sent to the evaporator (EVAP) (203) where heat is added to the pressurized water. Typically this is accomplished by burning a fuel in the boiler, and the heat of combustion is then transferred to the pressurized water that is routed through tubes and other passages and/or vessels in the boiler. As sufficient heat is added to the pressurized water, it boils and turns into steam (204). This steam now exists in the two-phase region where both steam and water coexist at the same pressure and temperature, called the saturation pressure and saturation temperature. For most applications designed in recent decades, this steam (204) is then sent to a superheater section (SHT) (205) in the boiler where it is heated to a higher temperature than saturation temperature. This steam (206) is now referred to as superheated steam. Superheated steam reduces (but does not eliminate) the risk of water carryover into the steam turbine (207), which is of concern since water carryover can cause extensive internal steam turbine damage. Of more importance, however, is the fact that superheated steam yields better cycle efficiencies. This is of great importance to large central power stations.
Once produced, the superheated steam (206) is sent to the steam turbine (207), typically via one or more pipes. The steam then begins to expand in the steam turbine (ST) and produce shaft horsepower. After traveling through the steam turbine down to a low exhaust pressure, the steam exits the ST (208), and is sent to the condenser (209), where it is then condensed back into water. This device is typically a tubed heat exchanger, but can also be other types of heat exchangers such as a spray chamber, air-cooled condenser, or other heat exchange device used for a similar purpose. After rejecting heat from the low-pressure steam and condensing the steam back to water, the condenser collects the water in an area commonly referred to as the hotwell (HW) (210), where it is then typically pumped through the condensate line (211) and back to the BFP (201). Shaft horsepower produced in the ST is converted into electrical power in the generator (GEN) (212). This cycle of one unit of water from the point of beginning, through the system, and back to the point of origin defines the basic Rankine Cycle.
Current power plants using only steam as the motive fluid typically use a boiler to produce the steam. This boiler may be fueled by a variety of fuels, including oil, natural gas, coal, biomass, as well as others, such as nuclear fuel. The boilers may also use a combination of fuels as well. Depending upon capital cost considerations, fuel costs, maintenance issues, and other factors, the owners and engineers will select the steam pressure and temperature at which the boiler will produce steam.
Due to the size and weight of large steam turbines, they require extended periods for start-up. This is due to the thick metal casings and large heavy rotors that are utilized in their construction. Therefore, these machines require long start-up periods to allow these heavy components to warm up uniformly, and avoid interference between stationary and rotating parts that may occur due to differential thermal expansion.
Although the heavy construction is a deterrent to rapid startup, it provides for robust construction and sustained performance levels. Even after four (4) years of nearly continuous service, the performance decay for a large ST should be less than 2%. This performance decay, combined with the fact that the boiler feed pumps only consume about 2% of the ST output, mean that the performance levels for a ST sustain near optimum levels for extended periods of time, even with decay in the auxiliary loads (BFP). In other words, if the BFP efficiency decays from 75% to 65%, the auxiliary load only increases from 2.00% to 2.31%. This is a small effect on the net output of the Rankine cycle plant, and is another one of its major advantages.
Brayton Cycle
The Brayton Cycle varies quite differently from the Rankine Cycle, as a major part of the cycle involves the compression of the working fluid, which is a compressible gas. This process consumes a great deal of power, therefore, efficient compression of the working fluid is essential to an efficient Brayton Cycle.
Common engines that utilize a Brayton Cycle are aircraft turboprops, jet engines, and gas turbines for stationary application. These engines work by ingesting air (the working fluid), compressing it to a higher pressure, typically 3 to 30 times that of the surrounding ambient air, adding heat through direct combustion (although heat addition from an external source is also possible), and then expanding the resulting high-pressure hot gases through a turbine section. Aircraft engines primarily produce thrust to propel an aircraft through the air. Therefore, some or perhaps none of their output is in the form of shaft horsepower (a turboprop gas turbine engine may drive the propeller, but may also produce some thrust from the high velocity exhaust gases).
For stationary gas turbine applications, the purpose of the engine is to produce shaft horsepower. Approximately ⅔ of the energy produced by the turbine section of the gas turbine is required to drive the compressor section, with the remaining ⅓ available to drive a load. This drawback of GT systems may be used to advantage in the present invention as described later in this document.
Aircraft engines utilize the Brayton Cycle because these engines offer high thrust-to-weight ratios. This is needed to minimize the aircraft weight so it can fly. For stationary applications, gas turbines are used to provide electrical power at peak loads. This is another advantage the Brayton Cycle engines have over Rankine Cycle engines: rapid start and stop times (relatively speaking). Since steam turbines are large heavy engines, it is necessary to start them slowly, and allow the heat to slowly soak into the thick casings so as to avoid thermal distortion and potential rubs between the stationary components and rotating components of the engine. A large power plant steam turbine may require a 24-hour warm-up sequence from cold start to reach full load. However, due to the lower operating pressures and lighter weights, gas turbines can be started and brought to fall load within a matter of minutes of start-up.
Therefore, many utilities in the United States and other countries use gas turbines to provide electrical power during peak demand. These turbines are not very efficient in simple cycle (25% to 30% LHV), but meet the electrical demand requirements for a few hours each day.
Steam Turbine Design
When designing a steam turbine for a power plant application (constant speed), the steam turbine design engineer first examines the output rating desired by the customer. This is because the steam turbine will be custom designed and manufactured for the customer to his specification. The steam turbine will not be totally designed from a clean sheet of paper as may be inferred by “custom”, but will utilize components from a “family” of hardware and have a unique steam path for the application. After turbine rating, the ST design engineer will look at the plant steam conditions, and based upon these parameters determine an inlet flow to the turbine high-pressure (HP) section. Utilizing this information, the ST design engineer can select the optimum HP casing for the application. In a similar fashion, he can also select the optimum intermediate pressure (IP) and low-pressure (LP) casings as well.
Knowing which casings to use, the engineer then selects the appropriate blading (both stationary and rotating) for the application. This blading size is determined primarily by the volume flow (as opposed to mass flow) of steam through the turbine. With casings and blading determined, the engineer completes the ST design by selecting valves, controls, instrumentation, and other accessories required for operation of the ST. The final design is a high efficiency ST optimized for the customer's steam conditions and desired rating.
An interesting note concerning this design philosophy is that two STs with the same steam conditions but with large differences in rating (for example, 200 MW versus 400 MW) may actually appear almost identical when viewed from the outside. This is because the optimum casings selected were designed to cover the flow range of both units. However, due to the large volume flow differences, the large unit would have blades that are approximately twice the size (height) internally. It is interesting to note, however, that both these units might have nearly the same HP and IP casings. This means that the larger ST, even with a dramatic increase in rating, may be only incrementally more expensive to manufacture than the ST with the lower rating. This fact may be used to advantage in the present invention as described later in this document.
Gas Turbine Design
Unlike the steam turbine, the gas turbine is not a custom designed machine for each customer. Although accessories such as the starting means, lube oil cooler type, and control options may be specified by the customer for a particular application, the core engine is essentially standard. Much of this is due to the fact that the gas turbine is actually a packaged power plant, which needs essentially only fuel to produce power. In contrast, the steam turbine is merely a component of a power plant, and requires a boiler, BFP, and condenser to become a complete power plant. Therefore, the gas turbine compressor section, combustion system, and turbine section must all be designed to work together. Since the design of the GT is a highly intensive engineering task, GT designs are generally completed and extensively tested, after which they are mass produced without variation to the core engine design. This eliminates the customer's ability to specify power output for either a facility with gas turbines only or a combined cycle facility in the prior art. When building a combined cycle plant, the customer simply must choose from a selection of standard offerings by a manufacturer that best meets his needs for power output, efficiency, and cost.
Steam Turbine/Gas Turbine Efficiency and Rating Comparison
The largest and most efficient GT available today for 60-cycle power production is rated at approximately 250 MW with an efficiency of 40.0% LHV (Lower Heating Value). An example of this GT is the Westinghouse model 501G. This is in contrast to STs that can be rated up to as high as 1500 MW and have overall cycle efficiencies in excess of 45% LHV. Therefore, comparing a Rankine Cycle power plant to a Brayton cycle power plant, where each employs the largest and most efficient turbine available, the single ST Rankine cycle is approximately six (6) times larger in rating and 12.5% more efficient than the Brayton Cycle with its best GT. This fact may be used to advantage in the present invention as described later in this document.
Cogeneration/Combined Cycle
One characteristic of the gas turbine is that it expels high volumes of exhaust gases at high temperature. With the advent of the Arab oil embargo of 1973 and higher energy prices, more focus was put on finding ways to utilize the energy contained in these high temperature exhaust gases.
Significantly higher energy prices in the early 1970s signaled the start of a wave of small power plants built using the principles of cogeneration. Cogeneration is defined as the simultaneous production of mechanical or electrical energy in conjunction with thermal energy. In other words, the utilization of an engine (gas turbine or otherwise) to produce power, while at the same time using waste heat from the engine for another process, thus displacing fuel that would otherwise be used for said process. This was a very efficient method from a fuel utilization perspective and was encouraged by the United States Public Utilities Regulation and Policies Act (PURPA) of 1978, which mandated that the local utilities must purchase power from qualified cogenerators, and buy it at a rate which included avoided cost for new power plants.
At first cogeneration projects were small, typically less than 50 MW. They consisted of small gas turbines with a HRSG to produce steam. In many instances, the steam pressures were relatively low (less than 600 psig), as the steam was used for process requirements. Some projects included a steam turbine, while others did not. As the industry matured, larger plants with higher steam pressures were designed to increase bottoming cycle efficiency. In addition, the major gas turbine manufacturers designed and built larger and more efficient gas turbines to meet the needs of the cogeneration marketplace. Soon, due to their high efficiency, low emissions, and low capital cost (dollars per kW of capacity), cogeneration power plants gave way to combined cycle power plants (plants that produced only power and provided no useful thermal energy as was the case with cogeneration plants). Some cogeneration projects are still being proposed and constructed, but they are now typically referred to as combined heat and power (CHP) projects.
Although there was this gradual shift from small cogeneration projects to large combined cycle power plants, the arrangement and overall system and method for producing power was for the most part unchanged. The gas turbine(s) was the primary engine, and a HRSG was utilized to capture the heat in the GT exhaust gases. Optimized for maximum power production, the steam turbine(s) produced additional power equal to approximately 50% of the power produced by the gas turbine(s). The HRSG was typically a two or three pressure level boiler to maximize heat recovery and steam turbine was designed to accept steam from all pressure levels of the HRSG. A review of the manufacturers standard combined cycle offerings will illustrate this trend. The 1997 TURBOMACHINERY HANDBOOK, (USPS 871-500, ISSN 0149-4147), tabulates standard combined cycle power plants available from various manufacturer's including ABB, General Electric, and Westinghouse. In most every instance, the steam turbine's output is within the range of 40% to 60% of the gas turbine(s) output. General Electric informative document GER-3567G, 1996, “GE Heavy-Duty Gas Turbine Performance Characteristics,” by Frank J. Brooks provides the output for the gas turbines used in their combined cycle power plants.
In summary, the system and method utilized by the major manufacturer's of combined cycle power plant turbomachinery evolved from the small cogeneration power facilities that were designed to produce both power and thermal energy simultaneously. The sizes for combined cycle power plants have grown from small cogeneration projects under 50 MW to large structured plants producing in excess of 700 MW (as in the Westinghouse 2X1 501G combined cycle). These plants are primarily gas turbine power plants, with the steam turbine producing additional power which is nominally 40% to 60% of the power produced by its associated gas turbine(s). With the gas turbine as the prime engine, the ratings on the standard combined cycle power plants are very rigid, as gas turbines are production line items, versus steam turbines which are largely custom designed and manufactured. A new system and method that offers more flexibility, without compromising the benefits of combined cycle power such as high efficiency, low emissions, and low capital cost, would be welcomed by the industry.